Full-scale evaluation of activated sludge thickening by dissolved air flotation

Abstract

This paper presents an overview of waste activated sludge thickening with dissolved air flotation (DAF). The most pertinent parameters that could influence DAF thickening performance are first identified, followed by a thorough review of the design models of Bratby and Marais (1975) and Bratby and Ambrose (1994) - the only rational theories available at this point. These theories are then tested against a comprehensive data set collected over a period of eight months during 22 site visits at five fullscale operating plants. The design models are poorly supported by the operational data. Black-box regression analyses are next performed on the data set to obtain estimates for sludge concentration, float-layer depth and clarity of the underflow. The regression analyses do not show strong correlation, but do demonstrate that, in three of the five plants visited, the float layers are too thin to withstand the disruptive action of the float scrapers. Without these disruptions, the existing models may very well be supported.

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